Fire is a natural component of forest and wildland ecosystems. Fires, whether caused by humans or lightning, can be devastating. Trees may be killed, shrubs providing food and cover for wildlife are reduced to ashes, and grasses or forest floors are burned away leaving the soil bare for the effects of erosion. However, fire used as a land management tool can be most beneficial. Prescribed burning is used to reduce the heavy buildup of fuels such as woody debris, provide nutrients to the soil, prepare areas for planting, restore tree stands, and create wildlife habitats.
Forest fires are greatly influenced by the moisture content of the forest floor. The forest floor is a nonhomogenous solid comprised of several layers. The top layer is the non-decomposed material consisting of twigs, grasses, leaves, pine needles and other dead debris. Next, is the duff region that is both a partially decomposed layer of organic material often bound with fungus, then an extensively decomposed layer. One may find many air voids in the duff region, which can vary in bulk density. The duff region also can vary in thickness from approximately two to twenty centimeters.
Fire can burn along the duff at either a slow or fast rate and the amount of duff region consumed by the fire is a function of the duff region's thickness and moisture content. An ideal system would allow managers to take real-time measurements of duff moisture content in the field. A portable moisture measurement system will aid land managers on the timing of prescribed burns, and aid wildfire managers in predicting fire behavior characteristics.
Duff moisture content is difficult to measure in the field using traditional techniques. Typically, a duff sample is taken in the field and sealed in an air-tight container. The sample is transported to a laboratory where it is weighed before being oven-dried at 85 degrees centigrade for 24 to 48 hours. Moisture content is then calculated using the field and oven-dry weights.
Time is critical when managers are faced with a wildfire situation, therefore, traditional duff moisture measurement methods taking at least 24 hours are inadequate. An instrument that can instantaneously provide duff moisture content greatly aids in the prediction of fire behavior as it relates to duff consumption. Effective fire behavior prediction would allow fire managers to better allocate fire-fighting resources to critical fire areas.
Currently, moisture content is not used in the predictive model for prescribed fire behavior because of the time it takes for measurement. However, real-time moisture content information would increase modeling accuracy in predicting the behavior of the prescribed burns and give more flexibility for users of this valuable conservation tool.
Moisture measurement circuitry, such as time domain reflectometry (TDR) and frequency domain impedance circuitry, operates by measuring the change in dielectric constant and is useful for moisture content measurements TDR measuring systems produce discrete electrical pulses that travel through and react with the medium resting against the electrodes. Basically, these electrical pulses travel at a velocity directly dependent on the dielectric constant of the surrounding medium; the higher the dielectric constant, the slower the electrical pulses will travel. This dielectric constant is dependent on the water and air void content of the measured medium. The measurement accuracy may also be affected by saline content, but in the application of duff measurement, this factor is unlikely to be present. Therefore, one can ascertain using time domain reflectometry techniques the changes in propagation velocity of the discrete electrical pulses traveling the length of the electrodes, and the velocity change can be directly correlated with the moisture content of the medium against which the electrodes rest.
Time domain reflectometry (TDR) has been used for moisture content measurements for years; therefore, prior inventions exist for the measurement of soil moisture content. (For example, refer to FIG. 1a depicting how a typical TDR probe is used for soil moisture content measurements.) However, prior inventions lack features that make these analysis techniques accurate for use with compressible materials such as forest duff.